This invention relates to apparatus for supporting a range of different loads or weights such that if the load is positioned manually at any elevation it will be counterpoised at that elevation.
The new load supporting apparatus will be exemplified herein in connection with devices for making x-ray diagnoses such as x-ray tube casings, x-ray image intensifiers and the like which are positioned manually with respect to an examination subject.
It is common practice to support an x-ray tube casing on an overhead suspension that is movable over an x-ray examination table. The casing is usually fastened to an extensible and contractible telescoping assembly which, along with the casing, is suspended from a steel cable. In some prior art arrangements, the casing is raised and lowered with a motor drive cable drum. In other arrangements, the casing is simply counterbalanced or counterpoised so that it will stay at whatever elevation it is set above a patient on an x-ray table.
A known type of manually operable counterpoise system uses a spirally wound flat spring steel strip which is inside of a container. A shaft extends through the center of the spiral for engaging the inner end of the spring. The outer end is fastened to a spirally groove generally axially tapered cam which is journaled on the shaft. The load is suspended on the cable which wraps around the cam and is fastened to the periphery of the cam. Means are provided to preload the spring until its torque just counterbalances the torque produced by the cable acting over the radius of the cam.
One of the problems with this rudimentary spring couterpoised system is that a different spring or a different cam is required for every different load. To meet the requirements of x-ray apparatus precisely, a dozen or more cams may be required. A compromise method for reducing the number of cams is to make one that is suitable for the maximum expected load and then to lead weights to any lighter load that is to be supported until the total weight equals the maximum in which case fairly satisfactory counterpoising will be obtained. In current practice, a wide variety of x-ray devices having different weights are used in different installations so it is still necessary, using conventional methods, to employ several different cams. However, the cost of the added weights, the cost of shipping them and the inconvenience of handling them still remains. Furthermore, any unnecessary weight is manifested as inertia in the system which means that more manual effort must be exerted to position the supported device at the desired elevation.
In some prior art spring counterpoise systems an attempt was made to compensate for using an improper or oversized cam by increasing the preloading of the spring. Winding the spring tighter than it was designed for, however, is at the expense of increasing friction between turns, thus, increasing hysteresis, leading to erratic operation and reduced life.
Another problem in prior counterpoise systems is uneven or eccentric winding of the torsion spring which leads to unduly high friction between turns which may result in overstress and fracture. Eccentric winding is due primarily to the conventional manner of anchoring the spring ends.
Still another problem in prior counterpoise systems resides in the devices which have been used to sense load cable and torsion spring failures. These devices usually depend on an auxiliary cable accepting the load when there is spring or load cable failure, but known devices lock-up when stress is temporarily removed from the load cable, such as due to the x-ray tube casing suddenly decelerating or coming to rest on another piece of equipment, and when this happens prior systems are difficult to reset. Furthermore, in many prior systems the auxiliary or safety cable carries a substantial portion of the load at all times and deteriorates almost as fast as the main load carrying cable.